18F-FDG PET/MRI was performed on a 67-year-old diabetic man referred for staging of a possible tumour revealed by multiple cranial nerve impairment (VII, IX, X, XI and XII) and skull-base osteolysis on CT. The PET image (a) shows heterogeneous uptake in the occipital bone and surrounding soft tissues (full arrow, SUVmax 5.0), with extension around the right condyle (dotted arrow). The contrast-enhanced T1-weighted MRI image (b) reveals hyperintensity in the clivus and the right occipital condyle/petrous apex, infiltrating contiguous soft tissues (full arrow), and focal posterior leptomeningeal enhancement (dotted arrow). The areas of hyperintensity coincide on the fused image (c) with the 18F-FDG uptake, and are clearly seen on the diffusion-weighted image (d). Linear 18F-FDG uptake in the right neurovascular structures is also shown on the coronal slices (e–g) and maximum intensity projection (h, arrowhead) with jugular and carotid compression on MRI. The association between moderate, diffuse 18F-FDG uptake and infiltrative contrast enhancement suggested osteomyelitis. The medical history indicated that this infection probably complicated a right otitis externa which occurred several weeks before and was treated with broad-spectrum antibiotics. Signs of active otitis are still visible on the PET/MRI images that show mild 18F-FDG uptake (stars, SUVmax 2.0) and contrast-enhanced wall thickening of the external auditory canal. The diagnosis was confirmed by an endonasal endoscopy biopsy carried out a few days later, that showed Pseudomonas aeruginosa and Stenotrophomonas maltophilia infections, and allowed an adapted antibacterial therapy to be started. This case illustrates the complementary value of PET and MRI for characterizing cell metabolism and the tissue microenvironment in osteomyelitis of the vertebrae [1] and skull base [2, 3] in a one-shot setting.

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